1. Field of the Invention
The present invention relates to polyisocyanate/polyamine mixtures in which the binder components possess improved compatibility with one another resulting in the production of coatings with improved optical properties, e.g., clarity.
2. Description of the Prior Art
One-component coating compositions which may be cured at room temperature are known and contain fully reacted polyurethanes as the binder. These compositions have the advantage that they are available as fully formulated systems which may be directly applied to suitable substrates without any preliminary steps except for mild stirring. Disadvantages of these systems are that large amounts of organic solvents are needed to reduce the viscosity of fully reacted, i.e., high molecular weight, polyurethanes and the polyurethanes must be essentially linear polyurethanes, which do not possess certain properties, e.g., solvent resistance, which may be obtained from crosslinked polyurethanes.
Two-component coating compositions are also known. These compositions come in two containers. The first contains a polyisocyanate, while the second contains an isocyanate-reactive component, generally a polyol. These systems do not require large amounts of solvent to obtain a suitable processing viscosity and can be used to obtain highly crosslinked coatings which possess properties that surpass those possessed by one-component coatings. However, these systems must be accurately mixed or the properties of the resulting coatings can be substantially affected. In addition, after the components are mixed they have a limited pot life since the components continue to react until an unusable solid is obtained.
Coating compositions which possess the advantages of the known one- and two-component coating compositions without possessing their disadvantages have been disclosed in copending applications, U.S. Ser. Nos. 08/171,281; 08/171,550; 08/171,304; 08/193,978; and 08/194,296. The coating compositions are prepared by blending polyisocyanates with certain aldimines. Even though coatings prepared in accordance with these copending applications possess many desirable properties, further improvements are needed in the appearance of the coatings, i.e., clarity, gloss and distinctness of image (DOI). These properties are directly related to the compatibility between the polyisocyanate and the aldimine.
Copending application, U.S. Ser. No. 08/171,281, discloses that the compatibility with aldimines may be improved by using certain allophanate group-containing polyisocyanates. However, there is still a need to achieve these improvements in compatibility and appearance with other commercially available polyisocyanates. In addition, even though the allophanate group-containing polyisocyanates described in the copending application are compatible with aldimines, they are not initially compatible with all aldimines, i.e., a cloudy solution may initially be formed. It may take 30 minutes or longer before a clear solution is obtained. In those applications in which it is desired to prepare a coating immediately after blending the components of the coating composition, it is necessary to improve the initial compatibility between the allophanate group-containing polyisocyanates and aldimines.
It is an object of the present invention to improve the compatibility, including the initial compatibility between polyisocyanates and aldimines so that the clarity, gloss and DOI of the resulting coatings is also improved. This object may be achieved with coating compositions according to the invention, which in addition to polyisocyanates and aldimines, also contain the polyaspartates described hereinafter.
U.S. Pat. Nos. 3,420,800 and 3,567,692 disclose coating compositions containing polyisocyanates and either aldimines or ketimines. U.S. Pat. No. 5,126,170 discloses coating compositions containing polyisocyanates, polyaspartates and optionally polyols. However, these patents do not teach that the compatibility of polyisocyanates with aldimines can be improved by the use of aldimine/aspartates of the present invention.
U.S. Pat. No. 5,214,086 discloses coating compositions containing polyisocyanates, aldimines, polyols and optionally polyaspartates. The examples of the present invention demonstrate that the coating compositions according to the invention possess superior properties when compared to the four component compositions disclosed by U.S. Pat. No. 5,214,086.
Copending application, U.S. Ser. No. 08/273,551, discloses that the compatibility of polyisocyanates with aldimines can be improved by also incorporating aspartates into the composition.